Magnetic recording media normally consist of a base film or support and a magnetic coating. The most widely used support in magnetic tape is poly-(ethylene terephthalate). The magnetic coatings normally contain, by volume, 40 to 55% magnetic material, usually a ferrimagnetic material, embedded in a binder matrix of about 30 to 35% of a polymeric material and small amounts of other additives such as lubricants, dispersants, conductive agents and the like. Of particular utility as organic polymer binders are the polyurethanes. In most present processes for manufacturing magnetic recording media, the polyurethane polymeric binder system is usually crosslinked chemically to improve stability and resistance to heat, decrease blocking and to provide enhanced life. In the case of polyurethane binders, this is usually accomplished by adding a polyfunctional isocyanate cross linker to the magnetic coating mix before deposition on the carrier. After deposition and drying to remove the solvent, the tape is wound or processed and the curing or crosslinking is usually accomplished by aging the rolls for 24 to 48 hours. A polyurethane binder that did not have to be cured or crosslinked by a polyisocyanate would be a great advantage. The variable isocyanate cure obtained because of the effect of even small amounts of moisture could be eliminated, and a faster curing system would provide for increased production rates. Thus, a polyurethane that can be quickly cured without the use of a polyisocyanate is desired.
Polymers not requiring a polyisocyanate cure are described, for example, in U.S. Pat. No. 4,255,243 disclosing photo-crosslinkable unsaturated poly-(carbonate-urethanes) obtained by condensing a diisocyanate with a polycarbonate-diol and a lower hydroxyalkyl acrylate. U.S. Pat. No. 4,260,703 discloses radiation curable urethane-acrylates prepared by reacting an alkoxylate of an ester diol such as 2,2-dimethyl-3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate with a diisocyanate, capped with a hydroxyalkyl acrylate such as hydroxyethyl acrylate. U.S. Pat. No. 4,264,752 discloses compositions formed by the reaction of a polycarbonate polyol, a polyisocyanate and a hydroxy acrylate monomer as hydroxyethyl acrylate that can be used as a component of radiation curable coatings. Also U.S. Pat. No. 4,254,230 discloses actinic radiation-curable unsaturated polyetherester urethanes in the presence of air. These are complex polymers containing at least one unsaturated urethane oligomer, said oligomer comprising the reaction product of at least one unsaturated active hydrogen-containing compound, at least one polyisocyanate; and at least one polyetherester. The amount of said unsaturated compound being present in amount molar equivalent to the isocyanate compound. None of these compounds are completely satisfactory as polyurethanes for electron beam curing for magnetic tape applications.